1. Field of the Invention
The invention relates to linearly reciprocating electric machines for converting mechanical to electrical, or electrical to mechanical power; and more particularly, to such machines having a constant, linear force versus current relation along the entire stroke, while providing higher efficiency with lower weight than machines of prior art types.
Long stroke motors of this type have been used for refrigerator drives and vibration absorbers, in particular for cryogenic cooling.
When used as generators, reciprocating linear stroke machines are particularly useful in long life unattended operations, especially if they are coupled to a prime mover which itself is a linear motion device.
Recent advances in the art of magnetic suspension or "magnetic bearings" make the combination of a Stirling cycle thermal-mechanical energy converter and a linear reciprocating mechanical-electrical energy converter a particularly happy combination. Use of these devices in an unattended satellite or a deep sea capsule requires that gears and lubricated joints be avoided if possible. At the same time, force or velocity harmonics should be avoided wherever possible because of the damage to this or other equipment which may be caused by attendant vibration. Thus any increase in the linearity of the operation of the machine is advantageous, because it simplifies control or balancing problems.
2. Description of the Prior Art
One of the earliest, and still most widely used, linear reciprocating motors is the common loudspeaker, which usually has a single layer coil wound to an accurate circular cylindrical shape, and mounted to fit precisely in the center of a cylindrical air gap across which a strong permanent magnetic field is established. Electrical current through the coil moves the coil along the gap axis with a force that is proportional to coil current, so long as the same number of turns of the coil are linked by the field flux. A fairly early form of such driver is disclosed in U.S. Pat. No. 1,921,924, which teaches improved performance through the use of an iron coil form around which the voice coil is wound. According to this patent, the use of the iron coil support or bobbin reduces the direct current excitation needed for the field magnet, and concentrates flux at the end of the voice coil nearer the field when the coil and form move away from the field itself. Because of the intention to concentrate as high a gap flux as possible, this patent teaches that the outer pole piece has an axial length which is substantially equal to that of the voice coil assembly. Therefore, any attempt to achieve a long stroke will cause the voice coil to move at least partly out of the gap.
British Pat. No. 568,682, which issued 12 years later, teaches a reduction in reluctance of the magnetic circuit by incorporating iron dust between the voice coil layers, particularly by mixing such dust into an insulating varnish or lacquer. Again, the voice coil and pole piece lengths are the same, so that on long strokes of the voice coil at least a portion of the coil is outside of the gap where the primary field flux is concentrated.
A related use of similar structures, but built to operate at far higher power levels, are the vibration exciters used for testing apparatus and calibrating vibration transducers or accelerometers. To provide maximum efficiency in the driver mechanism and good linearity, the stroke of these devices is held to a value which is small in comparison with the length of the driver coil. Where a longer stroke, while still maintaining driver linearity, is important, a most common approach is to use a relatively long driving coil, which extends beyond each end of the gap in the rest or center position. Then, during the entire operating stroke the entire length of the gap is occupied by turns of the coil, so that the percentage of coil length which is reacting with the field flux remains constant. Alternatively, as shown in German published patent application No. 2460551, published July 10, 1975, a magnetic gap structure may be provided which is much longer then the voice coil, so that over the entire stroke all of the coil is linked by the flux. This construction has a disadvantage that most of the energy in the field magnets is not utilized.
Thus, it is clear that the known moving coil designs have all suffered either from an efficiency loss, because there are some exposed coil turns during the stroke; or a non-linear response, because the number of coils within the gap varies over the stroke.
Where it has not been necessary to minimize the reciprocating mass, fixed coils with moving magnets have also been utilized for linear motors. This provides the great advantage that there is no need for coupling of electrical current into the moving part. However, these structures have also suffered the same difficulty that achievement of a long stroke is either achieved at the cost of a great nonlinearity of the force-current curve, or only a small part of the field flux in the air gap is actually utilized for motor action.